FeLV infection is a disease of veterinary importance able to induce multifaceted clinical signs as anemia, tumors, and immunodeficiency. In spite of effective vaccines, FeLV infection is still present but differs significantly in most study populations (Levy et al., 2006, J Am Vet Med Assoc. 228(3): 371-6).
After infection, FeLV first replicates in the local lymph nodes or the tonsils. Infected lymphocytes transport the virus to the bone marrow, where the virus replicates at high rates and finally spreads through the whole body via the blood stream. Integration of FeLV to the cells of the bone marrow and the blood renders a cat provirus positive. A cat becomes viremic if the virus is detectable in the blood. By infection of the mucosa of the bladder and/or the epithelia of the intestine and the salivary glands, FeLV can start a new cycle of infection.
Diagnosis of FeLV mainly relies on the detection of virus or virus antigen in the plasma, serum or whole blood. The most common serological tests detect either the presence of FeLV p27 antigen by enzyme-linked immunosorbent assay (ELISA) (Lutz et al. J. Immunol. Methods 56, 209-220 (1983), Lutz et al., Am J Vet Res. 44(11): 2054-9), or FeLV structural antigens in the cytoplasm of infected leukocytes and platelets by immunofluorescence antibody test (IFA; Hardy et al., 1991, J Am Vet Med Assoc. 199(10): 1327-35; ibid. 1365-73.). Moreover, western blot analysis detects the presence of FeLV antibodies. Alternatively, non-serological diagnoses include virus isolation (Jarrett O and Ganiere J P. Vet Rec. 138(1): 7-11), or the polymerase chain reaction (PCR) to detect proviral (FeLV DNA)—and viral load (FeLV RNA, Hofmann-Lehmann R et al. 2001. J Gen Virol. 82(Pt 7): 1589-96, Herring I P et al. 2001. Vet Ophthalmol. 4(2): 119-26, Jackson et al., 1996. J Vet Diagn Invest. 8(1): 25-30. Due to the time-consuming and/or cost-intensive character of most of these methods, they are not suitable for practical and clinical use. A serological test to detect FeLV antibodies for diagnosis has not been available until now, because no evidence has been provided regarding how reliable antibody detection is for predicting FeLV infection, and which antibodies are suitable.
FeLV disease outcomes in infected cats are rather unpredictable. They were termed abortive (provirus negative), progressive (persistently p27 positive, provirus positive, FeLV RNA positive, virus isolation positive), and regressive (p27 negative, provirus positive after or without transient antigenemia). It has however been reported (Gomes-Keller M A et al. 2009. Vet Microbiol. 134(3-4): 208-17; Major A et al. 2010. Vet Res. 41(2): 17.) that cats that may remain provirus negative in the blood but seroconvert, revealing that FeLV infection has occurred. Disease detection in these cats is detectable solely by a laborious and expensive serological procedure (immunofluorescence assay to feline oncornavirus-associated cell membrane antigen, ‘FOCMA-test’).
Until now, detection of antibodies to FeLV had limited significance as a means of diagnosis in veterinary practice, due to the widespread existence of endogenous FeLV (enFeLV) in cat populations. Because enFeLV is not tolerated by the immune system, antibodies are elicited (Langhammer et al. 2006. Immunology. 117(2): 229-37) which are undistinguishable from antibodies to exogenous FeLV. Only PCR is able to distinguish between endogenous and exogenous FeLV. Thus, FeLV antibodies were so far not considered to be useful as a diagnostic parameter. Moreover, several studies failed to detect a sufficient antibody response against various epitopes of FeLV. Fontenot and co-workers (J Clin Microbiol. (1992) 30(7): 1885-90) have analyzed the reactivity of a predicted FeLV transmembrane immunodominant domain (Imd-TM peptide, 26 amino acids long), and investigated its potential as a diagnostic reagent in serology. In the cited study, the peptide displays only negligible levels of reactivity using sera from FeLV-infected cats, which leads the authors to conclude that the Imd-TM peptide is not useful for FeLV diagnosis. Langhammer et al. (Immunology. 2006; 117(2): 229-37) recombinantly produced a FeLV p15E polypeptide covering the ectodomain (AA 476-583) and showed that cats infected with FeLV developed antibodies against p15E, although the reactions in ELISA were low. Epitope mapping revealed a variety of epitopes recognized by sera from FeLV-infected animals, including epitopes detected by sera from p15E-immunized cats, but the response in the infected animals was weaker than in vaccinated animals. The study concluded that natural FeLV infection results in a weak induction of antibodies specific for p15E, and a low induction of neutralizing antibodies.